||Water-quality problems in the Tualatin River Basin, Oregon, include low dissolved oxygen (DO), high pH, high water temperature, and high bacterial (Escherichia coli, or E. coli) counts, all of which episodically exceed State of Oregon water-quality standards. Excursions of pH typically are caused by algal blooms that grow in response to long travel times, warm water, and excessive nutrient inputs. In the reservoir system of the Tualatin River, low DO concentrations are most typically caused by sediment oxygen demand and long travel times in the absence of significant levels of photosynthesis and reaeration, although DO in the main stem also can be reduced by nitrification when ammonia concentrations are high (Rounds and Wood, 2001). In response to these and other water-quality problems, the State implemented Total Maximum Daily Loads (TMDLs) in 1988 for the Tualatin River Basin, as required under the Clean Water Act (Oregon Department of Environmental Quality, 1994a, 2001a). During 2001, the original phosphorus and ammonia TMDLs were revised, with new TMDLs added for water temperature, oxygenconsuming substances, and E. coli (Oregon Department of Environmental Quality, 2001b). In 1990, the U.S. Geological Survey (USGS) entered into a cooperative agreement with Clean Water Services (CWS - formerly the Unified Sewerage Agency) to investigate causes of water-quality problems in the river and evaluate alternatives for their management. Previous reports have described the TMDLs and USGS projects characterizing DO in the Tualatin River during winter (Kelly, 1997), nutrient sources and transport during low flows (Kelly and others, 1999), temperature modeling (Risley, 2000), sediment-oxygen demand (Rounds and Doyle, 1997), water-quality modeling (Rounds and Wood, 2001; Rounds and others, 1999), and phosphorus and bacteria in various tributaries during low-flow conditions (McCarthy, 2000). Technological improvements and programmatic changes have reduced loads of phosphorus and ammonia to the Tualatin River from point sources since 1991 (Rounds and Wood, 2001). However, because of continuing water-quality problems and ongoing urbanization, attention has increasingly turned to nonpoint sources for opportunities to further reduce contaminant loads. Tributary streams, which integrate nonpoint runoff from their entire watersheds, can be important transport pathways; however, water quality is a concern in some tributaries regardless of the effects on downstream receiving waters. Whereas the 1988 TMDL considered tributaries as a source of the phosphorus that was causing problems in the main stem, the 2001 TMDL focuses on problems in both the tributaries and the main stem. For instance, in the 2001 TMDL, Fanno Creek is allowed a summer median concentration of total phosphorus (TP) of 0.13 milligram per liter (mg/L) (Oregon Department of Environmental Quality, 2001a), and its phosphorus load during summer is considered part of the total 1,272 pounds allowed in the lower Tualatin during the same season. Thus, CWS and other resource managers are faced with the necessity of either controlling the concentration of TP in runoff or reducing the volume of runoff over the summer months. Loads of E. coli bacteria from point sources also are regulated on a seasonal basis, with higher cumulative concentrations allowed from these sources during summer storms (12,000 counts/100 mL) than during winter storms (5,000 counts/100 mL) (Oregon Department of Environmental Quality, 2001a). The State Standard for E. coli bacteria in a single, instantaneous stream sample is 406 counts/100 mL, or a monthly geometric mean of 126 colonies/100 mL for multiple samplings. Chlorophyll a concentrations in Fanno Creek occasionally exceed the State's action level of 15 micrograms per liter (µg/L) (Oregon Department of Environmental Quality, 2001a), although concentrations appear to have been decreasing in recent years (Jan Miller, Clean Water Services, written commun., April 2002). Total volatile suspended solids (TVSS) is regulated in the Tualatin River TMDL for control of sediment oxygen demand. McCarthy (2000) documented nutrients and bacteria concentrations in selected Tualatin River tributaries, including Fanno Creek, during summer low-flow conditions. Among her findings were that ground-water discharge could account for the phosphorus concentrations measured at most sites in the subbasin, but that local sources other than ground water were evident, possibly including avian waste materials and sediments resuspended from off-channel ponds. E. coli concentrations also were elevated at 70 percent of the sites sampled, possibly due to domestic pet and wildlife wastes, septic systems, or hobby farms. That study provided indications of the processes contributing nutrients during summer steady state, low-flow conditions, a period that is arguably the most sensitive regarding the effects of nutrients on eutrophication. Nonetheless, nutrients that enter creeks during other periods may be retained in the system, for example as particulate material in the bed sediments, and become biologically available during critical periods. Storm runoff is a significant process contributing sediment, nutrients, and bacteria to streams in urban areas, and likely provides part of the loads of these and other constituents to Fanno Creek. The purpose of this report is to characterize water quality, including sources and transport of nutrients and bacteria, during storm runoff conditions in the Fanno Creek Subbasin. Findings from this report ultimately will improve the understanding of dominant sources and transport processes in the basin and help improve water quality by strengthening the management of urban streams. During three storms from 1998 to 1999, data on nutrients, bacteria (E. coli), and constituents relating to their sources or transport (discharge, suspended solids) or their effects on water quality (biochemical oxygen demand, DO) were collected. Samples also were collected for analysis of trace elements and other inorganic constituents in water - data for those samples are stored in the CWS database but are not interpreted in this report. Multiple samples were collected at three sites during each storm, with the intent of characterizing conditions throughout individual storm hydrographs. Statistical relations among constituents are analyzed among all samplings, with exceptions unique to individual storms evaluated where they indicate important processes. Patterns and linkages from upstream to downstream also are explored.